1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube which can prevent deformation of a color selection electrode, such as a shadow mask, by adjusting the strength of a mask skirt portion.
2. Description of the Related Art
In a color cathode ray tube, for example, a shadow mask type color cathode ray tube used in a color television set, or a color display monitor for an OA (Office Automation) equipment terminal, a vacuum envelope is comprised of an approximately rectangular panel portion having a screen formed of a large number of phosphor films coated in a dot pattern or a stripe pattern on an inner surface of the panel portion, an approximately cylindrical neck portion which houses an electron gun and an approximately truncated-cone-shaped funnel portion which connects the neck portion and the panel portion. A color selection electrode (hereinafter referred to as “a shadow mask”) having a large number of electron beam apertures is closely spaced from the phosphor films within the vacuum envelope, and is fixed to a mask frame.
Generally aluminum killed steel is used as a constituent material of shadow masks, and along with the demand for high definition for color cathode ray tubes, shadow masks have also been used which are shaped by press-forming thin sheet metals. In color cathode ray tubes employing a shadow mask fabricated from a thin sheet metal, a phenomenon easily occurs in which a portion of the shadow mask is thermally deformed during operation and electron beam spots are displaced from their intended positions on a phosphor screen, and this phenomenon is referred to as “mask doming.” As countermeasures against such a phenomenon, along with improvement of a shadow mask suspension mechanism, Invar (a trademark, 36% Ni, 64% Fe) may also be used as a constituent material in view of its thermal expansion coefficient and physical hardness.
Such a shadow mask is fabricated by etching a large number of electron beam apertures through a blank sheet of about 0.1 to about 0.3 mm in thickness at specified positions, then stamping out a specified shape out of the blank sheet, and thereafter press-forming the blank sheet into a shape having an approximately spherical major surface and a skirt portion continuous with a periphery of the major surface an bent from the major surface in the tube axis direction. This shadow mask is welded and fixed to the mask frame as a shadow mask assembly, and is suspended from an inner wall of the panel portion.
FIG. 12 to FIG. 15 are explanatory views of one example of a shadow mask assembly in which a shadow mask and a mask frame are fixed together, wherein FIG. 12 is a side view of the shadow mask assembly, FIG. 13 is a plan view thereof, FIG. 14 is a cross-sectional view of an essential part of the shadow mask assembly showing positions where the shadow mask and the mask frame are fixed together, and FIG. 15 is a perspective view of a corner portion of the shadow mask assembly.
The shadow mask 5 is welded to the mask frame 6 at positions denoted by “x” marks of tongue portions 52 and the skirt portion 51, with the skirt portion 51, the tongue portions 52 and corner notches 58 fitted within the mask frame 6. In this configuration example, a spring 7 is welded to each side of the mask frame 6.
In the conventional shadow mask, its major surface and skirt portions are shaped by press-forming, spring back occurs in the press-formed skirt portion. In the skirt portion where the spring back has occurred, its end portion deflects in a direction away from the longitudinal axis of a cathode ray tube. There is a tendency that the amount of the deflection of the skirt portion is usually small at its corner portions where the degree of drawing by press-forming is relatively large, and the amount of the deflection of the skirt portion is large at central portions of its sides where the degree of drawing by press-forming is relatively small.
When the amount of the deflection of the skirt portion is increased, in operation of fitting the skirt portion into the support frame and then welding the fitted portion to the support frame, a large amount of the deflection hampers the fitting and welding operation, and consequently, workability in the fitting and welding operation is lowered.
Further, in the case of the conventional shadow mask, when the skirt portion having a large amount of the deflection is forcibly fitted into the support frame, a stress imparted to the fitted skirt portion is transmitted to non-apertured and apertured portions of the major surface and as a result a surface curvature of the apertured portion of the major surface of the shadow mask is deformed. Consequently, there arise problems such as lowering of the color selection performance of the shadow mask and lowering of the strength of the shadow mask.
Conventionally, various countermeasures have been taken for preventing the deformation of shadow masks. For example, there has been known a method which locally decreases a thickness of a peripheral portion of the major surface portion of the shadow mask. JP-A-9-35657 discloses a technique which copes with the deformation of the shadow mask by forming a plurality of stress absorbing holes in a skirt portion of a shadow mask. Further, many patent publications disclose various techniques which decrease the thickness of the shadow mask by forming non-through holes or grooves in regions ranging from peripheries of a major surface to a skirt portion.
Further, there has also been known a technique in which, for preventing landing errors attributed to the thermal expansion of a shadow mask, tongues are provided which project from the skirt portion substantially in parallel with the tube axis away from the major surface, and the tongues are fixed to the mask frame.
FIG. 16 and FIG. 17 are explanatory views of a typical conventional example of a shadow mask before press-forming and after press-forming. FIG. 16 is a plan view of a shadow mask blank before press-forming and FIG. 17 is a side view of the press-formed shadow mask. The shadow mask blank 5P includes a major surface 50 formed with electron beam apertures and skirt portions 51 and 54 around the major surface 50. The skirt portions 51 are arranged along long sides of the shadow mask 5 and the skirt portions 54 are arranged along short sides of the shadow mask 5. Tongue portions 52, 55 are formed in outer peripheries of respective central portions of the skirt portions 51 and 54, and corner notches 58 are formed between end portions of the skirt portion 51 and the skirt portion 54.
Formed in the skirt portions 51, 54 are elliptical openings 56, 57 larger compared to electron beam apertures formed in the major surface 50. These elliptical openings 56, 57 constitute a stress absorbing pattern which weakens the strength of the skirt portions 51, 54. The elliptical openings 56, 57 are made smaller with increasing distance from the tongue portions 52, 55. Due to the provision of these elliptical openings 56, 57, the strength of the skirt portions 51, 54 is weakened so that the deformation of he shadow mask 5 fixed to a mask frame can be suppressed. Here, the stress absorbing pattern is not limited to that of elliptical holes shown in FIG. 16, and may be formed of circular holes or slits.